1. Field of the Invention
This invention relates to a backside illuminated solid-state imaging device and a method for manufacturing the same.
2. Background Art
In recent years, the power consumption and size of solid-state imaging devices are being reduced in response to the popularity of digital still cameras and mobile telephones including cameras. CMOS (Complementary Metal Oxide Semiconductor) image sensors are being used in addition to conventional CCD (Charge-Coupled Device) image sensors. Such an image sensor is formed on one semiconductor chip and includes a sensor unit (imaging pixel unit), in which a plurality of pixels are two-dimensionally arranged, and a peripheral circuit unit disposed outside of the sensor unit.
In a conventional CMOS image sensor, a photodiode (PD) is formed in the surface of the silicon substrate for each pixel. An interconnect layer for transmitting electrical signals is provided on the silicon substrate. A color filter layer also is provided on the interconnect layer for each photodiode. A microlens is provided thereupon such that light efficiently reaches the photodiode. A glass substrate is disposed thereabove via a cavity.
In such a CMOS image sensor, light entering from the exterior passes through the glass substrate and the cavity, enters each microlens, is concentrated by the microlens, passes through the color filter layer and the interconnect layer, and then is incident on the photodiode. The light incident on the photodiode undergoes photoelectric conversion; a signal charge is produced; and an electrical signal is generated from the signal charge. Thereby, image data is acquired.
However, in such a CMOS image sensor, it is necessary that the light concentrated by the microlens passes through the interconnect layer including metal interconnect before reaching the photodiode. Although the metal interconnect is disposed in the interconnect layer to avoid regions between the microlenses and the photodiodes, the amount of light reflected/absorbed by the metal interconnect increases as pixel pitches are reduced due to downsizing of the image sensor and increasing pixel count. By this amount of reflected/absorbed light, the light incident on the photodiode decreases, and the light reception sensitivity decreases. The light reflected by the metal interconnect may become incident on a photodiode of an adjacent pixel to cause color mixture.
To solve such problems, a backside illuminated CMOS image sensor has been proposed (for example, refer to JP-A 2003-273343 (Kokai)). In a backside illuminated image sensor, a photodiode is formed in one face of a thin semiconductor substrate, and a color filter layer and a microlens are disposed on the one face. A glass substrate is disposed above the microlens via an adhesive layer and a cavity partitioned by the adhesive layer. On the other hand, transistors and the like forming a peripheral circuit are formed in another face of the semiconductor substrate. An interconnect layer is provided on the other face. A thick support substrate is affixed to the interconnect layer, and solder balls are mounted on the surface of the support substrate. Vias passing through the support substrate connect the solder balls to the metal interconnect in the interconnect layer.
In such a backside illuminated image sensor, optical components such as the photodiodes, the color filter layers, the microlenses, and the glass substrate are disposed on the side of one face of the semiconductor substrate; and electrical components such as the transistors forming the peripheral circuit, the interconnect layer, the vias, and the solder balls are disposed on the side of the other face of the semiconductor substrate. Therefore, the interconnect layer is not disposed in the optical path from the glass substrate through the cavity, the microlens, and the color filter layer to the photodiode. Therefore, light is not reflected/absorbed by the metal interconnect. As a result, light reception sensitivity is high, and color mixture does not occur readily.
However, it is necessary to provide a thick support substrate to ensure strength for such a backside illuminated image sensor. Therefore, the camera module height is undesirably increased by the amount of the support substrate and cannot easily be made thinner. Moreover, a process to make vias in the thick support substrate is necessary. Therefore, manufacturing costs undesirably increase.